Originally wire plant was developed to provide voice service using frequencies within the range of 300 Hz to 4 kHz. Explosive growth in internet and data traffic is necessitating two-way transmission of frequencies much higher than the currently available voice band, however physical limitations in the existing wire plant commonly used in subscriber circuits constrain this from happening. As replacing the extensive copper wire network with high bandwidth fibre optic cables would be prohibitively expensive, telecommunication companies prefer to exploit the current copper wire plant with new high frequency services such as ADSL.
Currently implemented copper wire plant exhibit antenna-like properties at high frequencies. As a result, EMI (electromagnetic interference) becomes a significant contributor to noise within the wire plant, particularly as distance increases, in effect reducing reliability and lowering transmission bandwidth and distance. When ADSL or ISDN is installed at a distant customer site, telephone companies often spend tremendous amounts of time and money troubleshooting noisy connections as a result of this interference.
A similar effect occurs with wires in close spatial proximity with each other, a phenomenon commonly referred to as crosstalk. Metallic signals on one pair cause longitudinal signals on itself and on neighboring pairs, causing increased metallic noise. This is often a cause of "ghost" voices in telephone conversations and the effect increases as frequency increases.
Furthermore, problems occur which are related to the mismatching of the longitudinal characteristic impedance of each wire. As a result of this mismatch, standing longitudinal waves are reflected by the discontinuity toward the source of the longitudinal voltage, further adding to the noise detailed above. This problem is more evident in telecommunications applications where long spans are the norm.
In U.S. Pat. No. 5,095,291 Staschover et al. teach of a communication filter for reducing noise on unshielded twisted pair cable for unidirectional PC LAN applications. However their solution only provides for single-ended termination, which provides small improvements on conductors used for bi-directional communications. In addition, the impedance to ground on the shunt transformer is very low, further exacerbating the effects of the above mentioned mismatching of the longitudinal impedance of the wire plant, particularly at high frequencies. As well, the negligible impedance to ground is unacceptable in telephony applications where telephones are biased at 48 V DC. In effect, the disadvantages caused by the longitudinal impedance mismatch may negate any benefits achieved by the reduction of noise signals.